1. Field of the Invention
The present invention relates generally to a kinetic pressure bearing motor, and more particularly, to a kinetic pressure bearing motor which is provided with an auxiliary dynamic pressure generating means, thus allowing a motor shaft to be stably rotated at a high speed and reducing vibration, therefore enhancing performance of the motor and reducing the manufacturing cost of the motor.
2. Description of the Prior Art
As well known to those skilled in the art, bearings are used in constant high-speed motors, such as a scanner motor which is used in a laser printer. The bearings function to support a motor shaft to prevent it from inclining when the motor is rotated at a high speed, so that the motor shaft stands upright. The conventional bearings are typically classified into two types, that is, a sintered oil retaining bearing and a fluid dynamic bearing.
FIG. 1 is a sectional view showing a conventional motor in which a sintered oil retaining bearing is installed. Referring to FIG. 1, the conventional motor having the sintered oil retaining bearing is provided with a cylindrical bearing holder 10. The bearing holder 10 has a hollow part in which a motor shaft 2 is inserted. An annular boss 10a is provided at an outer surface of the bearing holder 10. A base plate 9 is provided at the outer surface of the bearing holder 10 in such a way as to be supported at the annular boss 10a. 
Further, a stator assembly is mounted to the outer surface of the bearing holder 10, and is provided with a core 8a around which a coil 8 is wound.
The bearing holder 10 is designed such that the motor shaft 2 is rotatably inserted into the bearing holder 10. The motor shaft 2 is supported in vertical and horizontal directions by a sintered oil retaining bearing 1 which is provided in the bearing holder 10.
The sintered oil retaining bearing 1 is fabricated through a sintering process, so many inter-particle pores exist. Since oil is injected into the inter-particle pores in a vacuum state, oil is uniformly distributed in the bearing 1 at the initial rotating stage of the motor shaft 2 as well as when the motor shaft 2 is rotated or stopped.
A shaft holder 3 is fitted over an upper portion of the motor shaft 2. A polygon mirror 5 is firmly seated on an upper portion of the shaft holder 3.
A rotor casing 6 is mounted to a lower portion of the shaft holder 3. A magnet 7 is attached to an inner surface of the rotor casing 6 in such a way as to face the stator assembly. A predetermined gap is defined between the magnet 7 and the stator assembly.
The operation of the conventional motor with the sintered oil retaining bearing which is constructed in this way is as follows. First, when an electric current is applied to the coil 8 wound around the core 8a through the base panel 9 that is mounted to the outer surface of the bearing holder 10, electromagnetic force is generated between the core 8a, the coil 8, and the magnet 7 that is attached to the rotor casing 6. By the electromagnetic force, the magnet 7, the rotor casing 6, the shaft holder 3, and the polygon mirror 5 are simultaneously rotated around the motor shaft 2.
In this case, the sintered oil retaining bearing 1, provided at the outer surface of the motor shaft 2, allows the motor shaft 2 to be smoothly rotated while supporting the motor shaft 2.
The conventional motor with the sintered oil retaining bearing 1 is advantageous in that it is easy to manufacture, its manufacturing cost is low, and it has high durability.
However, the conventional motor with the sintered oil retaining bearing 1 has a problem that vibration may occur due to a gap between the motor shaft 2 and the bearing 1, so a speed variation is high and its life span becomes short.
Thus, in order to solve the problem, there have been proposed several methods to prevent the vibration. That is, a small gap of about 1˜3 μm may be defined between the sintered oil retaining bearing 1 and the motor shaft 2. Further, the sintered oil retaining bearing 1 may have an increased axial length.
However, such methods have a problem that periodic vibration is reduced but aperiodic vibration still remains. Further, when the gap between the sintered oil retaining bearing 1 and the motor shaft 2 is small, power consumption is undesirably increased and machining precision is degraded.
Meanwhile, FIG. 2 shows another conventional motor in which a fluid dynamic bearing is installed. As shown in FIG. 2, the conventional motor includes a cylindrical sleeve 20 which has a hollow part. A motor shaft 12 is inserted into the hollow part. A base panel 19 is mounted to an outer surface of the sleeve 20. A stator assembly 18 is mounted to the outer surface of an upper portion of the sleeve 20, and is provided with a core around which a coil is wound.
Further, ‘<’-shaped grooves 11 for generating dynamic pressure are formed on upper and lower portions of an outer surface of the motor shaft 12 which is inserted into the sleeve 20, through a cutting process.
In FIG. 2, the grooves 11 are formed on the outer surface of the motor shaft 12. However, the grooves 11 may be formed on an inner surface of the sleeve 20.
A shaft holder 13 is fitted over an upper portion of the motor shaft 12, and a polygon mirror 15 is firmly seated on an upper portion of the shaft holder 13. Further, a rotor casing 16 is mounted to a lower portion of the shaft holder 13. A magnet 17 is attached to an inner surface of the rotor casing 16 in such a way as to face the stator assembly 18. In this case, a predetermined gap is defined between the magnet 17 and the stator assembly 18.
In such a conventional fluid dynamic bearing motor, the magnet 17, the rotor casing 16, the shaft holder 13, and the polygon mirror 15 are simultaneously rotated around the motor shaft 12 by electromagnetic force which is generated between the core and the coil constituting the stator assembly 18, and the magnet 17 attached to the rotor casing 16. At this time, a laser beam is irradiated and reflected by the rotating polygon mirror 15.
In the conventional fluid dynamic bearing motor, oil (not shown) is spread over the grooves 11. Thus, as the motor shaft 12 is rotated, oil is concentrated into the grooves 11, thus generating oil pressure. The oil pressure allows the motor shaft 12 to be smoothly rotated without being in contact with the sleeve 20.
The conventional motor having the fluid dynamic bearing is advantageous in that it prevents vibration. However, the conventional motor having the fluid dynamic bearing is problematic in that the grooves 11 must be formed on the outer surface of the motor shaft 12 or the inner surface of the sleeve 20, so it is complicated to manufacture, and its manufacturing cost is higher in comparison with the motor having the sintered oil retaining bearing.
Further, the conventional motor having the fluid dynamic bearing has another problem that oil must be always supplied to the motors so as to generate dynamic pressure, so it is difficult to supply oil to the motor and oil may leak out.
Furthermore, when oil is short, expected dynamic pressure is not achieved. In this case, the motor shaft 12 frequently comes into frictional contact with the sleeve 20, so the motor's life span is reduced. Due to frictional contact between the motor shaft 12 and the sleeve 20, the motor shaft 12 may be thermally fused to the sleeve 20, so the motor may be undesirably stopped.